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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="manual-core-adv"></a>3. Using and understanding the Valgrind core: Advanced Topics</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="sect1"><a href="manual-core-adv.html#manual-core-adv.clientreq">3.1. The Client Request mechanism</a></span></dt>
<dt><span class="sect1"><a href="manual-core-adv.html#manual-core-adv.gdbserver">3.2. Debugging your program using Valgrind gdbserver and GDB</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-simple">3.2.1. Quick Start: debugging in 3 steps</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-concept">3.2.2. Valgrind gdbserver overall organisation</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-gdb">3.2.3. Connecting GDB to a Valgrind gdbserver</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-gdb-android">3.2.4. Connecting to an Android gdbserver</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-commandhandling">3.2.5. Monitor command handling by the Valgrind gdbserver</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-threads">3.2.6. Valgrind gdbserver thread information</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-shadowregisters">3.2.7. Examining and modifying Valgrind shadow registers</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.gdbserver-limitations">3.2.8. Limitations of the Valgrind gdbserver</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.vgdb">3.2.9. vgdb command line options</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.valgrind-monitor-commands">3.2.10. Valgrind monitor commands</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="manual-core-adv.html#manual-core-adv.wrapping">3.3. Function wrapping</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.example">3.3.1. A Simple Example</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.specs">3.3.2. Wrapping Specifications</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.semantics">3.3.3. Wrapping Semantics</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.debugging">3.3.4. Debugging</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.limitations-cf">3.3.5. Limitations - control flow</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.limitations-sigs">3.3.6. Limitations - original function signatures</a></span></dt>
<dt><span class="sect2"><a href="manual-core-adv.html#manual-core-adv.wrapping.examples">3.3.7. Examples</a></span></dt>
</dl></dd>
</dl>
</div>
<p>This chapter describes advanced aspects of the Valgrind core
services, which are mostly of interest to power users who wish to
customise and modify Valgrind's default behaviours in certain useful
ways.  The subjects covered are:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>The "Client Request" mechanism</p></li>
<li class="listitem"><p>Debugging your program using Valgrind's gdbserver
      and GDB</p></li>
<li class="listitem"><p>Function Wrapping</p></li>
</ul></div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="manual-core-adv.clientreq"></a>3.1. The Client Request mechanism</h2></div></div></div>
<p>Valgrind has a trapdoor mechanism via which the client
program can pass all manner of requests and queries to Valgrind
and the current tool.  Internally, this is used extensively 
to make various things work, although that's not visible from the
outside.</p>
<p>For your convenience, a subset of these so-called client
requests is provided to allow you to tell Valgrind facts about
the behaviour of your program, and also to make queries.
In particular, your program can tell Valgrind about things that it
otherwise would not know, leading to better results.
</p>
<p>Clients need to include a header file to make this work.
Which header file depends on which client requests you use.  Some
client requests are handled by the core, and are defined in the
header file <code class="filename">valgrind/valgrind.h</code>.  Tool-specific
header files are named after the tool, e.g.
<code class="filename">valgrind/memcheck.h</code>.  Each tool-specific header file
includes <code class="filename">valgrind/valgrind.h</code> so you don't need to
include it in your client if you include a tool-specific header.  All header
files can be found in the <code class="literal">include/valgrind</code> directory of
wherever Valgrind was installed.</p>
<p>The macros in these header files have the magical property
that they generate code in-line which Valgrind can spot.
However, the code does nothing when not run on Valgrind, so you
are not forced to run your program under Valgrind just because you
use the macros in this file.  Also, you are not required to link your
program with any extra supporting libraries.</p>
<p>The code added to your binary has negligible performance impact:
on x86, amd64, ppc32, ppc64 and ARM, the overhead is 6 simple integer
instructions and is probably undetectable except in tight loops.
However, if you really wish to compile out the client requests, you
can compile with <code class="option">-DNVALGRIND</code> (analogous to
<code class="option">-DNDEBUG</code>'s effect on
<code class="function">assert</code>).
</p>
<p>You are encouraged to copy the <code class="filename">valgrind/*.h</code> headers
into your project's include directory, so your program doesn't have a
compile-time dependency on Valgrind being installed.  The Valgrind headers,
unlike most of the rest of the code, are under a BSD-style license so you may
include them without worrying about license incompatibility.</p>
<p>Here is a brief description of the macros available in
<code class="filename">valgrind.h</code>, which work with more than one
tool (see the tool-specific documentation for explanations of the
tool-specific macros).</p>
<div class="variablelist"><dl class="variablelist">
<dt><span class="term"><span class="command"><strong><code class="computeroutput">RUNNING_ON_VALGRIND</code></strong></span>:</span></dt>
<dd><p>Returns 1 if running on Valgrind, 0 if running on the
    real CPU.  If you are running Valgrind on itself, returns the
    number of layers of Valgrind emulation you're running on.
    </p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_DISCARD_TRANSLATIONS</code>:</strong></span></span></dt>
<dd>
<p>Discards translations of code in the specified address
    range.  Useful if you are debugging a JIT compiler or some other
    dynamic code generation system.  After this call, attempts to
    execute code in the invalidated address range will cause
    Valgrind to make new translations of that code, which is
    probably the semantics you want.  Note that code invalidations
    are expensive because finding all the relevant translations
    quickly is very difficult, so try not to call it often.
    Note that you can be clever about
    this: you only need to call it when an area which previously
    contained code is overwritten with new code.  You can choose
    to write code into fresh memory, and just call this
    occasionally to discard large chunks of old code all at
    once.</p>
<p>
    Alternatively, for transparent self-modifying-code support,
    use<code class="option">--smc-check=all</code>, or run
    on ppc32/Linux, ppc64/Linux or ARM/Linux.
    </p>
</dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_COUNT_ERRORS</code>:</strong></span></span></dt>
<dd><p>Returns the number of errors found so far by Valgrind.  Can be
    useful in test harness code when combined with the
    <code class="option">--log-fd=-1</code> option; this runs Valgrind silently,
    but the client program can detect when errors occur.  Only useful
    for tools that report errors, e.g. it's useful for Memcheck, but for
    Cachegrind it will always return zero because Cachegrind doesn't
    report errors.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_MALLOCLIKE_BLOCK</code>:</strong></span></span></dt>
<dd><p>If your program manages its own memory instead of using
    the standard <code class="function">malloc</code> /
    <code class="function">new</code> /
    <code class="function">new[]</code>, tools that track
    information about heap blocks will not do nearly as good a
    job.  For example, Memcheck won't detect nearly as many
    errors, and the error messages won't be as informative.  To
    improve this situation, use this macro just after your custom
    allocator allocates some new memory.  See the comments in
    <code class="filename">valgrind.h</code> for information on how to use
    it.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_FREELIKE_BLOCK</code>:</strong></span></span></dt>
<dd><p>This should be used in conjunction with
    <code class="computeroutput">VALGRIND_MALLOCLIKE_BLOCK</code>.
    Again, see <code class="filename">valgrind.h</code> for
    information on how to use it.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_RESIZEINPLACE_BLOCK</code>:</strong></span></span></dt>
<dd><p>Informs a Valgrind tool that the size of an allocated block has been
    modified but not its address. See <code class="filename">valgrind.h</code> for
    more information on how to use it.</p></dd>
<dt><span class="term">
   <span class="command"><strong><code class="computeroutput">VALGRIND_CREATE_MEMPOOL</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_DESTROY_MEMPOOL</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_MEMPOOL_ALLOC</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_MEMPOOL_FREE</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_MOVE_MEMPOOL</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_MEMPOOL_CHANGE</code></strong></span>,
   <span class="command"><strong><code class="computeroutput">VALGRIND_MEMPOOL_EXISTS</code></strong></span>:
   </span></dt>
<dd><p>These are similar to 
    <code class="computeroutput">VALGRIND_MALLOCLIKE_BLOCK</code> and
    <code class="computeroutput">VALGRIND_FREELIKE_BLOCK</code>
    but are tailored towards code that uses memory pools.  See 
    <a class="xref" href="mc-manual.html#mc-manual.mempools" title="4.8. Memory Pools: describing and working with custom allocators">Memory Pools</a> for a detailed description.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_NON_SIMD_CALL[0123]</code>:</strong></span></span></dt>
<dd>
<p>Executes a function in the client program on the
    <span class="emphasis"><em>real</em></span> CPU, not the virtual CPU that Valgrind
    normally runs code on.  The function must take an integer (holding a
    thread ID) as the first argument and then 0, 1, 2 or 3 more arguments
    (depending on which client request is used).  These are used in various
    ways internally to Valgrind.  They might be useful to client
    programs.</p>
<p><span class="command"><strong>Warning:</strong></span> Only use these if you
    <span class="emphasis"><em>really</em></span> know what you are doing.  They aren't 
    entirely reliable, and can cause Valgrind to crash.  See
    <code class="filename">valgrind.h</code> for more details.
    </p>
</dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_PRINTF(format, ...)</code>:</strong></span></span></dt>
<dd><p>Print a printf-style message to the Valgrind log file.  The
    message is prefixed with the PID between a pair of
    <code class="computeroutput">**</code> markers.  (Like all client requests,
    nothing is output if the client program is not running under Valgrind.)
    Output is not produced until a newline is encountered, or subsequent
    Valgrind output is printed; this allows you to build up a single line of
    output over multiple calls.  Returns the number of characters output,
    excluding the PID prefix.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_PRINTF_BACKTRACE(format, ...)</code>:</strong></span></span></dt>
<dd><p>Like <code class="computeroutput">VALGRIND_PRINTF</code> (in
    particular, the return value is identical), but prints a stack backtrace
    immediately afterwards.</p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_MONITOR_COMMAND(command)</code>:</strong></span></span></dt>
<dd><p>Execute the given monitor command (a string).
    Returns 0 if command is recognised. Returns 1 if command is not recognised.
    Note that some monitor commands provide access to a functionality
    also accessible via a specific client request. For example,
    memcheck leak search can be requested from the client program
    using VALGRIND_DO_LEAK_CHECK or via the monitor command "leak_search".
    Note that the syntax of the command string is only verified at
    run-time. So, if it exists, it is preferable to use a specific
    client request to have better compile time verifications of the
    arguments.
    </p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_CLO_CHANGE(option)</code>:</strong></span></span></dt>
<dd><p>Changes the value of a dynamically changeable option (a string).
       See <a class="xref" href="manual-core.html#manual-core.dynopts" title="2.7.8. Dynamically Changing Options">Dynamically Change Options</a>.
    </p></dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_STACK_REGISTER(start, end)</code>:</strong></span></span></dt>
<dd>
<p>Registers a new stack.  Informs Valgrind that the memory range
    between start and end is a unique stack.  Returns a stack identifier
    that can be used with other
    <code class="computeroutput">VALGRIND_STACK_*</code> calls.</p>
<p>Valgrind will use this information to determine if a change
    to the stack pointer is an item pushed onto the stack or a change
    over to a new stack.  Use this if you're using a user-level thread
    package and are noticing crashes in stack trace recording or
    spurious errors from Valgrind about uninitialized memory
    reads.</p>
<p><span class="command"><strong>Warning:</strong></span> Unfortunately, this client request is
    unreliable and best avoided.</p>
</dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_STACK_DEREGISTER(id)</code>:</strong></span></span></dt>
<dd>
<p>Deregisters a previously registered stack.  Informs
    Valgrind that previously registered memory range with stack id
    <code class="computeroutput">id</code> is no longer a stack.</p>
<p><span class="command"><strong>Warning:</strong></span> Unfortunately, this client request is
    unreliable and best avoided.</p>
</dd>
<dt><span class="term"><span class="command"><strong><code class="computeroutput">VALGRIND_STACK_CHANGE(id, start, end)</code>:</strong></span></span></dt>
<dd>
<p>Changes a previously registered stack.  Informs
    Valgrind that the previously registered stack with stack id
    <code class="computeroutput">id</code> has changed its start and end
    values.  Use this if your user-level thread package implements
    stack growth.</p>
<p><span class="command"><strong>Warning:</strong></span> Unfortunately, this client request is
    unreliable and best avoided.</p>
</dd>
</dl></div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="manual-core-adv.gdbserver"></a>3.2. Debugging your program using Valgrind gdbserver and GDB</h2></div></div></div>
<p>A program running under Valgrind is not executed directly by the
CPU.  Instead it runs on a synthetic CPU provided by Valgrind.  This is
why a debugger cannot debug your program when it runs on Valgrind.
</p>
<p>
This section describes how GDB can interact with the
Valgrind gdbserver to provide a fully debuggable program under
Valgrind. Used in this way, GDB also provides an interactive usage of
Valgrind core or tool functionalities, including incremental leak search
under Memcheck and on-demand Massif snapshot production.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-simple"></a>3.2.1. Quick Start: debugging in 3 steps</h3></div></div></div>
<p>The simplest way to get started is to run Valgrind with the
flag <code class="option">--vgdb-error=0</code>.  Then follow the on-screen
directions, which give you the precise commands needed to start GDB
and connect it to your program.</p>
<p>Otherwise, here's a slightly more verbose overview.</p>
<p>If you want to debug a program with GDB when using the Memcheck
tool, start Valgrind like this:
</p>
<pre class="screen">
valgrind --vgdb=yes --vgdb-error=0 prog
</pre>
<p>In another shell, start GDB:
</p>
<pre class="screen">
gdb prog
</pre>
<p>Then give the following command to GDB:
</p>
<pre class="screen">
(gdb) target remote | vgdb
</pre>
<p>You can now debug your program e.g. by inserting a breakpoint
and then using the GDB <code class="computeroutput">continue</code>
command.</p>
<p>This quick start information is enough for basic usage of the
Valgrind gdbserver.  The sections below describe more advanced
functionality provided by the combination of Valgrind and GDB. Note
that the command line flag <code class="option">--vgdb=yes</code> can be omitted,
as this is the default value.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-concept"></a>3.2.2. Valgrind gdbserver overall organisation</h3></div></div></div>
<p>The GNU GDB debugger is typically used to debug a process
running on the same machine.  In this mode, GDB uses system calls to
control and query the program being debugged.  This works well, but
only allows GDB to debug a program running on the same computer.
</p>
<p>GDB can also debug processes running on a different computer.
To achieve this, GDB defines a protocol (that is, a set of query and
reply packets) that facilitates fetching the value of memory or
registers, setting breakpoints, etc.  A gdbserver is an implementation
of this "GDB remote debugging" protocol.  To debug a process running
on a remote computer, a gdbserver (sometimes called a GDB stub)
must run at the remote computer side.
</p>
<p>The Valgrind core provides a built-in gdbserver implementation,
which is activated using <code class="option">--vgdb=yes</code>
or <code class="option">--vgdb=full</code>.  This gdbserver allows the process
running on Valgrind's synthetic CPU to be debugged remotely.
GDB sends protocol query packets (such as "get register contents") to
the Valgrind embedded gdbserver.  The gdbserver executes the queries
(for example, it will get the register values of the synthetic CPU)
and gives the results back to GDB.
</p>
<p>GDB can use various kinds of channels (TCP/IP, serial line, etc)
to communicate with the gdbserver.  In the case of Valgrind's
gdbserver, communication is done via a pipe and a small helper program
called <a class="xref" href="manual-core-adv.html#manual-core-adv.vgdb" title="3.2.9. vgdb command line options">vgdb</a>, which acts as an
intermediary.  If no GDB is in use, vgdb can also be
used to send monitor commands to the Valgrind gdbserver from a shell
command line.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-gdb"></a>3.2.3. Connecting GDB to a Valgrind gdbserver</h3></div></div></div>
<p>To debug a program "<code class="filename">prog</code>" running under
Valgrind, you must ensure that the Valgrind gdbserver is activated by
specifying either <code class="option">--vgdb=yes</code>
or <code class="option">--vgdb=full</code>.  A secondary command line option,
<code class="option">--vgdb-error=number</code>, can be used to tell the gdbserver
only to become active once the specified number of errors have been
shown.  A value of zero will therefore cause
the gdbserver to become active at startup, which allows you to
insert breakpoints before starting the run.  For example:
</p>
<pre class="screen">
valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog
</pre>
<p>The Valgrind gdbserver is invoked at startup
and indicates it is waiting for a connection from a GDB:</p>
<pre class="programlisting">
==2418== Memcheck, a memory error detector
==2418== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2418== Using Valgrind-3.14.0.GIT and LibVEX; rerun with -h for copyright info
==2418== Command: ./prog
==2418== 
==2418== (action at startup) vgdb me ... 
</pre>
<p>GDB (in another shell) can then be connected to the Valgrind gdbserver.
For this, GDB must be started on the program <code class="filename">prog</code>:
</p>
<pre class="screen">
gdb ./prog
</pre>
<p>You then indicate to GDB that you want to debug a remote target:
</p>
<pre class="screen">
(gdb) target remote | vgdb
</pre>
<p>
GDB then starts a vgdb relay application to communicate with the 
Valgrind embedded gdbserver:</p>
<pre class="programlisting">
(gdb) target remote | vgdb
Remote debugging using | vgdb
relaying data between gdb and process 2418
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2418]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb) 
</pre>
<p>Note that vgdb is provided as part of the Valgrind
distribution.  You do not need to install it separately.</p>
<p>If vgdb detects that there are multiple Valgrind gdbservers that
can be connected to, it will list all such servers and their PIDs, and
then exit.  You can then reissue the GDB "target" command, but
specifying the PID of the process you want to debug:
</p>
<pre class="programlisting">
(gdb) target remote | vgdb
Remote debugging using | vgdb
no --pid= arg given and multiple valgrind pids found:
use --pid=2479 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2481 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2483 for valgrind --vgdb=yes --vgdb-error=0 ./another_prog 
Remote communication error: Resource temporarily unavailable.
(gdb)  target remote | vgdb --pid=2479
Remote debugging using | vgdb --pid=2479
relaying data between gdb and process 2479
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2479]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb) 
</pre>
<p>Once GDB is connected to the Valgrind gdbserver, it can be used
in the same way as if you were debugging the program natively:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Breakpoints can be inserted or deleted.</p></li>
<li class="listitem"><p>Variables and register values can be examined or modified.
    </p></li>
<li class="listitem"><p>Signal handling can be configured (printing, ignoring).
    </p></li>
<li class="listitem"><p>Execution can be controlled (continue, step, next, stepi, etc).
    </p></li>
<li class="listitem"><p>Program execution can be interrupted using Control-C.</p></li>
</ul></div>
<p>And so on.  Refer to the GDB user manual for a complete
description of GDB's functionality.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-gdb-android"></a>3.2.4. Connecting to an Android gdbserver</h3></div></div></div>
<p> When developping applications for Android, you will typically use
a development system (on which the Android NDK is installed) to compile your
application. An Android target system or emulator will be used to run
the application.
In this setup, Valgrind and vgdb will run on the Android system,
while GDB will run on the development system. GDB will connect
to the vgdb running on the Android system using the Android NDK
'adb forward' application.
</p>
<p> Example: on the Android system, execute the following:
    </p>
<pre class="screen">
valgrind --vgdb-error=0 --vgdb=yes prog
# and then in another shell, run:
vgdb --port=1234
</pre>
<p>
</p>
<p> On the development system, execute the following commands:
</p>
<pre class="screen">
adb forward tcp:1234 tcp:1234
gdb prog
(gdb) target remote :1234
</pre>
<p>
GDB will use a local tcp/ip connection to connect to the Android adb forwarder.
Adb will establish a relay connection between the host system and the Android
target system.  Be sure to use the GDB delivered in the
Android NDK system (typically, arm-linux-androideabi-gdb), as the host
GDB is probably not able to debug Android arm applications.
Note that the local port nr (used by GDB) must not necessarily be equal
to the port number used by vgdb: adb can forward tcp/ip between different
port numbers.
</p>
<p>In the current release, the GDB server is not enabled by default
for Android, due to problems in establishing a suitable directory in
which Valgrind can create the necessary FIFOs (named pipes) for
communication purposes.  You can stil try to use the GDB server, but
you will need to explicitly enable it using the flag 
<code class="computeroutput">--vgdb=yes</code> or
<code class="computeroutput">--vgdb=full</code>.
</p>
<p>Additionally, you
will need to select a temporary directory which is (a) writable
by Valgrind, and (b) supports FIFOs.  This is the main difficult
point.  Often, <code class="computeroutput">/sdcard</code> satisfies
requirement (a), but fails for (b) because it is a VFAT file system
and VFAT does not support pipes.  Possibilities you could try are
<code class="computeroutput">/data/local</code>,
<code class="computeroutput">/data/local/Inst</code> (if you
installed Valgrind there), or
<code class="computeroutput">/data/data/name.of.my.app</code>, if you
are running a specific application and it has its own directory of 
that form.  This last possibility may have the highest probability
of success.</p>
<p>You can specify the temporary directory to use either via
the <code class="computeroutput">--with-tmpdir=</code> configure time
flag, or by setting environment variable TMPDIR when running Valgrind
(on the Android device, not on the Android NDK development host).
Another alternative is to specify the directory for the FIFOs using
the <code class="computeroutput">--vgdb-prefix=</code> Valgrind command
line option.
</p>
<p>We hope to have a better story for temporary directory handling
on Android in the future.  The difficulty is that, unlike in standard
Unixes, there is no single temporary file directory that reliably
works across all devices and scenarios.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-commandhandling"></a>3.2.5. Monitor command handling by the Valgrind gdbserver</h3></div></div></div>
<p> The Valgrind gdbserver provides additional Valgrind-specific
functionality via "monitor commands".  Such monitor commands can be
sent from the GDB command line or from the shell command line or
requested by the client program using the VALGRIND_MONITOR_COMMAND
client request.  See
<a class="xref" href="manual-core-adv.html#manual-core-adv.valgrind-monitor-commands" title="3.2.10. Valgrind monitor commands">Valgrind monitor commands</a> for the
list of the Valgrind core monitor commands available regardless of the
Valgrind tool selected.
</p>
<p>The following tools provide tool-specific monitor commands:
  </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><a class="xref" href="mc-manual.html#mc-manual.monitor-commands" title="4.6. Memcheck Monitor Commands">Memcheck Monitor Commands</a></p></li>
<li class="listitem"><p><a class="xref" href="cl-manual.html#cl-manual.monitor-commands" title="6.4. Callgrind Monitor Commands">Callgrind Monitor Commands</a></p></li>
<li class="listitem"><p><a class="xref" href="ms-manual.html#ms-manual.monitor-commands" title="9.5. Massif Monitor Commands">Massif Monitor Commands</a></p></li>
<li class="listitem"><p><a class="xref" href="hg-manual.html#hg-manual.monitor-commands" title="7.7. Helgrind Monitor Commands">Helgrind Monitor Commands</a></p></li>
</ul></div>
<p>
</p>
<p>An example of a tool specific monitor command is the Memcheck monitor
command <code class="computeroutput">leak_check full
reachable any</code>.  This requests a full reporting of the
allocated memory blocks.  To have this leak check executed, use the GDB
command:
</p>
<pre class="screen">
(gdb) monitor leak_check full reachable any
</pre>
<p>
</p>
<p>GDB will send the <code class="computeroutput">leak_check</code>
command to the Valgrind gdbserver.  The Valgrind gdbserver will
execute the monitor command itself, if it recognises it to be a Valgrind core
monitor command.  If it is not recognised as such, it is assumed to
be tool-specific and is handed to the tool for execution.  For example:
</p>
<pre class="programlisting">
(gdb) monitor leak_check full reachable any
==2418== 100 bytes in 1 blocks are still reachable in loss record 1 of 1
==2418==    at 0x4006E9E: malloc (vg_replace_malloc.c:236)
==2418==    by 0x804884F: main (prog.c:88)
==2418== 
==2418== LEAK SUMMARY:
==2418==    definitely lost: 0 bytes in 0 blocks
==2418==    indirectly lost: 0 bytes in 0 blocks
==2418==      possibly lost: 0 bytes in 0 blocks
==2418==    still reachable: 100 bytes in 1 blocks
==2418==         suppressed: 0 bytes in 0 blocks
==2418== 
(gdb) 
</pre>
<p>As with other GDB commands, the Valgrind gdbserver will accept
abbreviated monitor command names and arguments, as long as the given
abbreviation is unambiguous.  For example, the above
<code class="computeroutput">leak_check</code>
command can also be typed as:
</p>
<pre class="screen">
(gdb) mo l f r a
</pre>
<p>

The letters <code class="computeroutput">mo</code> are recognised by GDB as being
an abbreviation for <code class="computeroutput">monitor</code>.  So GDB sends the
string <code class="computeroutput">l f r a</code> to the Valgrind
gdbserver.  The letters provided in this string are unambiguous for the
Valgrind gdbserver.  This therefore gives the same output as the
unabbreviated command and arguments.  If the provided abbreviation is
ambiguous, the Valgrind gdbserver will report the list of commands (or
argument values) that can match:
</p>
<pre class="programlisting">
(gdb) mo v. n
v. can match v.set v.info v.wait v.kill v.translate v.do
(gdb) mo v.i n
n_errs_found 0 n_errs_shown 0 (vgdb-error 0)
(gdb) 
</pre>
<p>
</p>
<p>Instead of sending a monitor command from GDB, you can also send
these from a shell command line.  For example, the following command
lines, when given in a shell, will cause the same leak search to be executed
by the process 3145:
</p>
<pre class="screen">
vgdb --pid=3145 leak_check full reachable any
vgdb --pid=3145 l f r a
</pre>
<p>Note that the Valgrind gdbserver automatically continues the
execution of the program after a standalone invocation of
vgdb.  Monitor commands sent from GDB do not cause the program to
continue: the program execution is controlled explicitly using GDB
  commands such as "continue" or "next".</p>
<p>Many monitor commands (e.g. v.info location, memcheck who_points_at, ...) require an address
  argument and an optional length: <code class="varname">&lt;addr&gt; [&lt;len&gt;]</code>.
  The arguments can also be provided by using a 'C array like syntax' by providing the address
  followed by the length between square brackets.</p>
<p>For example, the following two monitor commands provide the same information:</p>
<pre class="programlisting">
(gdb) mo xb 0x804a2f0 10
...
(gdb) mo xb 0x804a2f0[10]
...
</pre>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-threads"></a>3.2.6. Valgrind gdbserver thread information</h3></div></div></div>
<p>Valgrind's gdbserver enriches the output of the
GDB <code class="computeroutput">info threads</code> command
with Valgrind-specific information.
The operating system's thread number is followed
by Valgrind's internal index for that thread ("tid") and by
the Valgrind scheduler thread state:</p>
<pre class="programlisting">
(gdb) info threads
  4 Thread 6239 (tid 4 VgTs_Yielding)  0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
* 3 Thread 6238 (tid 3 VgTs_Runnable)  make_error (s=0x8048b76 "called from London") at prog.c:20
  2 Thread 6237 (tid 2 VgTs_WaitSys)  0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
  1 Thread 6234 (tid 1 VgTs_Yielding)  main (argc=1, argv=0xbedcc274) at prog.c:105
(gdb) 
</pre>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-shadowregisters"></a>3.2.7. Examining and modifying Valgrind shadow registers</h3></div></div></div>
<p> When the option <code class="option">--vgdb-shadow-registers=yes</code> is
given, the Valgrind gdbserver will let GDB examine and/or modify
Valgrind's shadow registers.  GDB version 7.1 or later is needed for this
to work. For x86 and amd64, GDB version 7.2 or later is needed.</p>
<p>For each CPU register, the Valgrind core maintains two
shadow register sets.  These shadow registers can be accessed from
GDB by giving a postfix <code class="computeroutput">s1</code>
or <code class="computeroutput">s2</code> for respectively the first
and second shadow register.  For example, the x86 register
<code class="computeroutput">eax</code> and its two shadows
can be examined using the following commands:</p>
<pre class="programlisting">
(gdb) p $eax
$1 = 0
(gdb) p $eaxs1
$2 = 0
(gdb) p $eaxs2
$3 = 0
(gdb) 
</pre>
<p>Float shadow registers are shown by GDB as unsigned integer
values instead of float values, as it is expected that these
shadow values are mostly used for memcheck validity bits. </p>
<p>Intel/amd64 AVX registers <code class="computeroutput">ymm0</code>
to <code class="computeroutput">ymm15</code> have also their shadow
registers. However, GDB presents the shadow values using two
"half" registers. For example, the half shadow registers for 
<code class="computeroutput">ymm9</code> are
<code class="computeroutput">xmm9s1</code> (lower half for set 1),
<code class="computeroutput">ymm9hs1</code> (upper half for set 1),
<code class="computeroutput">xmm9s2</code> (lower half for set 2),
<code class="computeroutput">ymm9hs2</code> (upper half for set 2).
Note the inconsistent notation for the names of the half registers:
the lower part starts with an <code class="computeroutput">x</code>,
the upper part starts with an <code class="computeroutput">y</code>
and has an <code class="computeroutput">h</code> before the shadow postfix.
</p>
<p>The special presentation of the AVX shadow registers is due to
the fact that GDB independently retrieves the lower and upper half of
the <code class="computeroutput">ymm</code> registers.  GDB does not
however know that the shadow half registers have to be shown combined.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.gdbserver-limitations"></a>3.2.8. Limitations of the Valgrind gdbserver</h3></div></div></div>
<p>Debugging with the Valgrind gdbserver is very similar to native
debugging.  Valgrind's gdbserver implementation is quite
complete, and so provides most of the GDB debugging functionality.  There
are however some limitations and peculiarities:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>Precision of "stop-at" commands.</p>
<p>
       GDB commands such as "step", "next", "stepi", breakpoints
       and watchpoints, will stop the execution of the process.  With
       the option <code class="option">--vgdb=yes</code>, the process might not
       stop at the exact requested instruction. Instead, it might
       continue execution of the current basic block and stop at one
       of the following basic blocks. This is linked to the fact that
       Valgrind gdbserver has to instrument a block to allow stopping
       at the exact instruction requested.  Currently,
       re-instrumentation of the block currently being executed is not
       supported. So, if the action requested by GDB (e.g. single
       stepping or inserting a breakpoint) implies re-instrumentation
       of the current block, the GDB action may not be executed
       precisely.
     </p>
<p>
       This limitation applies when the basic block
       currently being executed has not yet been instrumented for debugging.
       This typically happens when the gdbserver is activated due to the
       tool reporting an error or to a watchpoint.  If the gdbserver
       block has been activated following a breakpoint, or if a
       breakpoint has been inserted in the block before its execution,
       then the block has already been instrumented for debugging.
     </p>
<p>
       If you use the option <code class="option">--vgdb=full</code>, then GDB
       "stop-at" commands will be obeyed precisely.  The
       downside is that this requires each instruction to be
       instrumented with an additional call to a gdbserver helper
       function, which gives considerable overhead (+500% for memcheck)
       compared to  <code class="option">--vgdb=no</code>.
       Option <code class="option">--vgdb=yes</code> has neglectible overhead compared
       to <code class="option">--vgdb=no</code>.
     </p>
</li>
<li class="listitem">
<p>Processor registers and flags values.</p>
<p>When Valgrind gdbserver stops on an error, on a breakpoint
     or when single stepping, registers and flags values might not be always
     up to date due to the optimisations done by the Valgrind core.
     The default value 
     <code class="option">--vex-iropt-register-updates=unwindregs-at-mem-access</code>
     ensures that the registers needed to make a stack trace (typically
     PC/SP/FP) are up to date at each memory access (i.e. memory exception
     points).
     Disabling some optimisations using the following values will increase
     the precision of registers and flags values (a typical performance 
     impact for memcheck is given for each option).
       </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p>
           <code class="option">--vex-iropt-register-updates=allregs-at-mem-access</code> (+10%)
           ensures that all registers and flags are up to date at each memory
           access.
         </p></li>
<li class="listitem"><p>
           <code class="option">--vex-iropt-register-updates=allregs-at-each-insn</code> (+25%)
           ensures that all registers and flags are up to date at each instruction.
         </p></li>
</ul></div>
<p>
       Note that <code class="option">--vgdb=full</code> (+500%, see above
       Precision of "stop-at" commands) automatically
       activates <code class="option">--vex-iropt-register-updates=allregs-at-each-insn</code>.
     </p>
</li>
<li class="listitem">
<p>Hardware watchpoint support by the Valgrind
     gdbserver.</p>
<p> The Valgrind gdbserver can simulate hardware watchpoints
     if the selected tool provides support for it.  Currently,
     only Memcheck provides hardware watchpoint simulation.  The
     hardware watchpoint simulation provided by Memcheck is much
     faster that GDB software watchpoints, which are implemented by
     GDB checking the value of the watched zone(s) after each
     instruction.  Hardware watchpoint simulation also provides read
     watchpoints.  The hardware watchpoint simulation by Memcheck has
     some limitations compared to real hardware
     watchpoints. However, the number and length of simulated
     watchpoints are not limited.
     </p>
<p>Typically, the number of (real) hardware watchpoints is
     limited.  For example, the x86 architecture supports a maximum of
     4 hardware watchpoints, each watchpoint watching 1, 2, 4 or 8
     bytes. The Valgrind gdbserver does not have any limitation on the
     number of simulated hardware watchpoints. It also has no
     limitation on the length of the memory zone being
     watched.  Using GDB version 7.4 or later allow full use of the
     flexibility of the Valgrind gdbserver's simulated hardware watchpoints.
     Previous GDB versions do not understand that Valgrind gdbserver
     watchpoints have no length limit.
     </p>
<p>Memcheck implements hardware watchpoint simulation by
     marking the watched address ranges as being unaddressable.  When
     a hardware watchpoint is removed, the range is marked as
     addressable and defined.  Hardware watchpoint simulation of
     addressable-but-undefined memory zones works properly, but has
     the undesirable side effect of marking the zone as defined when
     the watchpoint is removed.
     </p>
<p>Write watchpoints might not be reported at the 
     exact instruction that writes the monitored area,
     unless option <code class="option">--vgdb=full</code> is given.  Read watchpoints
     will always be reported at the exact instruction reading the
     watched memory.
     </p>
<p>It is better to avoid using hardware watchpoint of not
     addressable (yet) memory: in such a case, GDB will fall back to
     extremely slow software watchpoints.  Also, if you do not quit GDB
     between two debugging sessions, the hardware watchpoints of the
     previous sessions will be re-inserted as software watchpoints if
     the watched memory zone is not addressable at program startup.
     </p>
</li>
<li class="listitem">
<p>Stepping inside shared libraries on ARM.</p>
<p>For unknown reasons, stepping inside shared
     libraries on ARM may fail.  A workaround is to use the
     <code class="computeroutput">ldd</code> command
     to find the list of shared libraries and their loading address
     and inform GDB of the loading address using the GDB command
     "add-symbol-file". Example:
     </p>
<pre class="programlisting">
(gdb) shell ldd ./prog
	libc.so.6 =&gt; /lib/libc.so.6 (0x4002c000)
	/lib/ld-linux.so.3 (0x40000000)
(gdb) add-symbol-file /lib/libc.so.6 0x4002c000
add symbol table from file "/lib/libc.so.6" at
	.text_addr = 0x4002c000
(y or n) y
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.
(gdb) 
</pre>
<p>
     </p>
</li>
<li class="listitem">
<p>GDB version needed for ARM and PPC32/64.</p>
<p>You must use a GDB version which is able to read XML
     target description sent by a gdbserver.  This is the standard setup
     if GDB was configured and built with the "expat"
     library.  If your GDB was not configured with XML support, it
     will report an error message when using the "target"
     command.  Debugging will not work because GDB will then not be
     able to fetch the registers from the Valgrind gdbserver.
     For ARM programs using the Thumb instruction set, you must use
     a GDB version of 7.1 or later, as earlier versions have problems
     with next/step/breakpoints in Thumb code.
     </p>
</li>
<li class="listitem">
<p>Stack unwinding on PPC32/PPC64. </p>
<p>On PPC32/PPC64, stack unwinding for leaf functions
     (functions that do not call any other functions) works properly
     only when you give the option
     <code class="option">--vex-iropt-register-updates=allregs-at-mem-access</code>
     or <code class="option">--vex-iropt-register-updates=allregs-at-each-insn</code>.
     You must also pass this option in order to get a precise stack when
     a signal is trapped by GDB.
     </p>
</li>
<li class="listitem">
<p>Breakpoints encountered multiple times.</p>
<p>Some instructions (e.g. x86 "rep movsb")
     are translated by Valgrind using a loop.  If a breakpoint is placed
     on such an instruction, the breakpoint will be encountered
     multiple times -- once for each step of the "implicit" loop
     implementing the instruction.
     </p>
</li>
<li class="listitem">
<p>Execution of Inferior function calls by the Valgrind
     gdbserver.</p>
<p>GDB allows the user to "call" functions inside the process
     being debugged.  Such calls are named "inferior calls" in the GDB
     terminology.  A typical use of an inferior call is to execute
     a function that prints a human-readable version of a complex data
     structure.  To make an inferior call, use the GDB "print" command
     followed by the function to call and its arguments.  As an
     example, the following GDB command causes an inferior call to the
     libc "printf" function to be executed by the process
     being debugged:
     </p>
<pre class="programlisting">
(gdb) p printf("process being debugged has pid %d\n", getpid())
$5 = 36
(gdb) 
</pre>
<p>The Valgrind gdbserver supports inferior function calls.
     Whilst an inferior call is running, the Valgrind tool will report
     errors as usual.  If you do not want to have such errors stop the
     execution of the inferior call, you can
     use <code class="computeroutput">v.set vgdb-error</code> to set a
     big value before the call, then manually reset it to its original
     value when the call is complete.</p>
<p>To execute inferior calls, GDB changes registers such as
     the program counter, and then continues the execution of the
     program. In a multithreaded program, all threads are continued,
     not just the thread instructed to make the inferior call.  If
     another thread reports an error or encounters a breakpoint, the
     evaluation of the inferior call is abandoned.</p>
<p>Note that inferior function calls are a powerful GDB
     feature, but should be used with caution. For example, if
     the program being debugged is stopped inside the function "printf",
     forcing a recursive call to printf via an inferior call will
     very probably create problems.  The Valgrind tool might also add
     another level of complexity to inferior calls, e.g. by reporting
     tool errors during the Inferior call or due to the
     instrumentation done.
     </p>
</li>
<li class="listitem">
<p>Connecting to or interrupting a Valgrind process blocked in
     a system call.</p>
<p>Connecting to or interrupting a Valgrind process blocked in
     a system call requires the "ptrace" system call to be usable.
     This may be disabled in your kernel for security reasons.
     </p>
<p>When running your program, Valgrind's scheduler
     periodically checks whether there is any work to be handled by
     the gdbserver.  Unfortunately this check is only done if at least
     one thread of the process is runnable.  If all the threads of the
     process are blocked in a system call, then the checks do not
     happen, and the Valgrind scheduler will not invoke the gdbserver.
     In such a case, the vgdb relay application will "force" the
     gdbserver to be invoked, without the intervention of the Valgrind
     scheduler.
     </p>
<p>Such forced invocation of the Valgrind gdbserver is
     implemented by vgdb using ptrace system calls.  On a properly
     implemented kernel, the ptrace calls done by vgdb will not
     influence the behaviour of the program running under Valgrind.
     If however they do, giving the
     option <code class="option">--max-invoke-ms=0</code> to the vgdb relay
     application will disable the usage of ptrace calls.  The
     consequence of disabling ptrace usage in vgdb is that a Valgrind
     process blocked in a system call cannot be woken up or
     interrupted from GDB until it executes enough basic blocks to let
     the Valgrind scheduler's normal checking take effect.
     </p>
<p>When ptrace is disabled in vgdb, you can increase the
     responsiveness of the Valgrind gdbserver to commands or
     interrupts by giving a lower value to the
     option <code class="option">--vgdb-poll</code>.  If your application is
     blocked in system calls most of the time, using a very low value
     for <code class="option">--vgdb-poll</code> will cause a the gdbserver to be
     invoked sooner.  The gdbserver polling done by Valgrind's
     scheduler is very efficient, so the increased polling frequency
     should not cause significant performance degradation.
     </p>
<p>When ptrace is disabled in vgdb, a query packet sent by GDB
     may take significant time to be handled by the Valgrind
     gdbserver.  In such cases, GDB might encounter a protocol
     timeout.  To avoid this,
     you can increase the value of the timeout by using the GDB
     command "set remotetimeout".
     </p>
<p>Ubuntu versions 10.10 and later may restrict the scope of
     ptrace to the children of the process calling ptrace.  As the
     Valgrind process is not a child of vgdb, such restricted scoping
     causes the ptrace calls to fail.  To avoid that, Valgrind will
     automatically allow all processes belonging to the same userid to
     "ptrace" a Valgrind process, by using PR_SET_PTRACER.</p>
<p>Unblocking processes blocked in system calls is not
     currently implemented on Mac OS X and Android.  So you cannot
     connect to or interrupt a process blocked in a system call on Mac
     OS X or Android.
     </p>
<p>Unblocking processes blocked in system calls is implemented
     via agent thread on Solaris. This is quite a different approach
     than using ptrace on Linux, but leads to equivalent result - Valgrind
     gdbserver is invoked. Note that agent thread is a Solaris OS
     feature and cannot be disabled.
     </p>
</li>
<li class="listitem">
<p>Changing register values.</p>
<p>The Valgrind gdbserver will only modify the values of the
     thread's registers when the thread is in status Runnable or
     Yielding.  In other states (typically, WaitSys), attempts to
     change register values will fail.  Amongst other things, this
     means that inferior calls are not executed for a thread which is
     in a system call, since the Valgrind gdbserver does not implement
     system call restart.
     </p>
</li>
<li class="listitem">
<p>Unsupported GDB functionality.</p>
<p>GDB provides a lot of debugging functionality and not all
     of it is supported.  Specifically, the following are not
     supported: reversible debugging and tracepoints.
     </p>
</li>
<li class="listitem">
<p>Unknown limitations or problems.</p>
<p>The combination of GDB, Valgrind and the Valgrind gdbserver
     probably has unknown other limitations and problems.  If you
     encounter strange or unexpected behaviour, feel free to report a
     bug.  But first please verify that the limitation or problem is
     not inherent to GDB or the GDB remote protocol.  You may be able
     to do so by checking the behaviour when using standard gdbserver
     part of the GDB package.
     </p>
</li>
</ul></div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.vgdb"></a>3.2.9. vgdb command line options</h3></div></div></div>
<p> Usage: <code class="computeroutput">vgdb [OPTION]... [[-c] COMMAND]...</code></p>
<p> vgdb ("Valgrind to GDB") is a small program that is used as an
intermediary between Valgrind and GDB or a shell.
Therefore, it has two usage modes:
</p>
<div class="orderedlist">
<a name="vgdb.desc.modes"></a><ol class="orderedlist" type="1">
<li class="listitem"><p><a name="manual-core-adv.vgdb-standalone"></a>As a standalone utility, it is used from a shell command
    line to send monitor commands to a process running under
    Valgrind. For this usage, the vgdb OPTION(s) must be followed by
    the monitor command to send. To send more than one command,
    separate them with the <code class="option">-c</code> option.
    </p></li>
<li class="listitem"><p><a name="manual-core-adv.vgdb-relay"></a>In combination with GDB "target remote |" command, it is
    used as the relay application between GDB and the Valgrind
    gdbserver.  For this usage, only OPTION(s) can be given, but no
    COMMAND can be given.
    </p></li>
</ol>
</div>
<p><code class="computeroutput">vgdb</code> accepts the following
options:</p>
<div class="variablelist">
<a name="vgdb.opts.list"></a><dl class="variablelist">
<dt><span class="term"><code class="option">--pid=&lt;number&gt;</code></span></dt>
<dd><p>Specifies the PID of
    the process to which vgdb must connect to.  This option is useful
    in case more than one Valgrind gdbserver can be connected to.  If
    the <code class="option">--pid</code> argument is not given and multiple
    Valgrind gdbserver processes are running, vgdb will report the
    list of such processes and then exit.</p></dd>
<dt><span class="term"><code class="option">--vgdb-prefix</code></span></dt>
<dd><p>Must be given to both
    Valgrind and vgdb if you want to change the default prefix for the
    FIFOs (named pipes) used for communication between the Valgrind
    gdbserver and vgdb.</p></dd>
<dt><span class="term"><code class="option">--wait=&lt;number&gt;</code></span></dt>
<dd><p>Instructs vgdb to
    search for available Valgrind gdbservers for the specified number
    of seconds.  This makes it possible start a vgdb process 
    before starting the Valgrind gdbserver with which you intend the
    vgdb to communicate.  This option is useful when used in
    conjunction with a <code class="option">--vgdb-prefix</code> that is
    unique to the process you want to wait for.
    Also, if you use the <code class="option">--wait</code> argument in the GDB
    "target remote" command, you must set the GDB remotetimeout to a
    value bigger than the --wait argument value.  See option
    <code class="option">--max-invoke-ms</code> (just below)
    for an example of setting the remotetimeout value.</p></dd>
<dt><span class="term"><code class="option">--max-invoke-ms=&lt;number&gt;</code></span></dt>
<dd>
<p>Gives the
    number of milliseconds after which vgdb will force the invocation
    of gdbserver embedded in Valgrind.  The default value is 100
    milliseconds. A value of 0 disables forced invocation. The forced
    invocation is used when vgdb is connected to a Valgrind gdbserver,
    and the Valgrind process has all its threads blocked in a system
    call.
    </p>
<p>If you specify a large value, you might need to increase the
    GDB "remotetimeout" value from its default value of 2 seconds.
    You should ensure that the timeout (in seconds) is
    bigger than the <code class="option">--max-invoke-ms</code> value.  For
    example, for <code class="option">--max-invoke-ms=5000</code>, the following
    GDB command is suitable:
    </p>
<pre class="screen">
    (gdb) set remotetimeout 6
    </pre>
<p>
    </p>
</dd>
<dt><span class="term"><code class="option">--cmd-time-out=&lt;number&gt;</code></span></dt>
<dd><p>Instructs a
    standalone vgdb to exit if the Valgrind gdbserver it is connected
    to does not process a command in the specified number of seconds.
    The default value is to never time out.</p></dd>
<dt><span class="term"><code class="option">--port=&lt;portnr&gt;</code></span></dt>
<dd>
<p>Instructs vgdb to
    use tcp/ip and listen for GDB on the specified port nr rather than
    to use a pipe to communicate with GDB. Using tcp/ip allows to have
    GDB running on one computer and debugging a Valgrind process
    running on another target computer.
    Example: 
    </p>
<pre class="screen">
# On the target computer, start your program under valgrind using
valgrind --vgdb-error=0 prog
# and then in another shell, run:
vgdb --port=1234
</pre>
<p>On the computer which hosts GDB, execute the command:
    </p>
<pre class="screen">
gdb prog
(gdb) target remote targetip:1234
</pre>
<p>
    where targetip is the ip address or hostname of the target computer.
    </p>
</dd>
<dt><span class="term"><code class="option">-c</code></span></dt>
<dd>
<p>To give more than one command to a
    standalone vgdb, separate the commands by an
    option <code class="option">-c</code>. Example:
    </p>
<pre class="screen">
vgdb v.set log_output -c leak_check any
</pre>
</dd>
<dt><span class="term"><code class="option">-l</code></span></dt>
<dd><p>Instructs a standalone vgdb to report
    the list of the Valgrind gdbserver processes running and then
    exit.</p></dd>
<dt><span class="term"><code class="option">-T</code></span></dt>
<dd><p>Instructs vgdb to add timestamps to vgdb
    information messages.
    </p></dd>
<dt><span class="term"><code class="option">-D</code></span></dt>
<dd><p>Instructs a standalone vgdb to show the
    state of the shared memory used by the Valgrind gdbserver.  vgdb
    will exit after having shown the Valgrind gdbserver shared memory
    state.</p></dd>
<dt><span class="term"><code class="option">-d</code></span></dt>
<dd><p>Instructs vgdb to produce debugging
    output.  Give multiple <code class="option">-d</code> args to increase the
    verbosity. When giving <code class="option">-d</code> to a relay vgdb, you better
    redirect the standard error (stderr) of vgdb to a file to avoid
    interaction between GDB and vgdb debugging output.</p></dd>
</dl>
</div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.valgrind-monitor-commands"></a>3.2.10. Valgrind monitor commands</h3></div></div></div>
<p>This section describes the Valgrind monitor commands, available
regardless of the Valgrind tool selected. For the tool specific
commands, refer to <a class="xref" href="mc-manual.html#mc-manual.monitor-commands" title="4.6. Memcheck Monitor Commands">Memcheck Monitor Commands</a>,
<a class="xref" href="hg-manual.html#hg-manual.monitor-commands" title="7.7. Helgrind Monitor Commands">Helgrind Monitor Commands</a>,
<a class="xref" href="cl-manual.html#cl-manual.monitor-commands" title="6.4. Callgrind Monitor Commands">Callgrind Monitor Commands</a> and
<a class="xref" href="ms-manual.html#ms-manual.monitor-commands" title="9.5. Massif Monitor Commands">Massif Monitor Commands</a>. </p>
<p> The monitor commands can be sent either from a shell command line, by using a
standalone vgdb, or from GDB, by using GDB's "monitor"
command (see <a class="xref" href="manual-core-adv.html#manual-core-adv.gdbserver-commandhandling" title="3.2.5. Monitor command handling by the Valgrind gdbserver">Monitor command handling by the Valgrind gdbserver</a>).
They can also be launched by the client program, using the VALGRIND_MONITOR_COMMAND
client request.
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><code class="varname">help [debug]</code> instructs Valgrind's gdbserver
    to give the list of all monitor commands of the Valgrind core and
    of the tool. The optional "debug" argument tells to also give help
    for the monitor commands aimed at Valgrind internals debugging.
    </p></li>
<li class="listitem"><p><code class="varname">v.info all_errors</code> shows all errors found
    so far.</p></li>
<li class="listitem"><p><code class="varname">v.info last_error</code> shows the last error
    found.</p></li>
<li class="listitem">
<p><code class="varname">v.info location &lt;addr&gt;</code> outputs
    information about the location &lt;addr&gt;. Possibly, the
    following are described: global variables, local (stack)
    variables, allocated or freed blocks, ...  The information
    produced depends on the tool and on the options given to valgrind.
    Some tools (e.g. memcheck and helgrind) produce more detailed
    information for client heap blocks. For example, these tools show
    the stacktrace where the heap block was allocated. If a tool does
    not replace the malloc/free/... functions, then client heap blocks
    will not be described.  Use the
    option <code class="varname">--read-var-info=yes</code> to obtain more
    detailed information about global or local (stack) variables.
    </p>
<pre class="programlisting">
(gdb) monitor v.info location 0x8050b20
 Location 0x8050b20 is 0 bytes inside global var "mx"
 declared at tc19_shadowmem.c:19

(gdb) mo v.in loc 0x582f33c
 Location 0x582f33c is 0 bytes inside local var "info"
 declared at tc19_shadowmem.c:282, in frame #1 of thread 3
(gdb) 
</pre>
</li>
<li class="listitem"><p><code class="varname">v.info n_errs_found [msg]</code> shows the number of
    errors found so far, the nr of errors shown so far and the current
    value of the <code class="option">--vgdb-error</code> argument. The optional
    <code class="computeroutput">msg</code> (one or more words) is appended.
    Typically, this can be used to insert markers in a process output
    file between several tests executed in sequence by a process
    started only once. This allows to associate the errors reported
    by Valgrind with the specific test that produced these errors.
    </p></li>
<li class="listitem"><p><code class="varname">v.info open_fds</code> shows the list of open file
    descriptors and details related to the file descriptor.
    This only works if <code class="option">--track-fds=yes</code> or
    <code class="option">--track-fds=all</code> (to include
    <code class="computeroutput">stdin</code>,
    <code class="computeroutput">stdout</code> and
    <code class="computeroutput">stderr</code>) was given at Valgrindr
    startup.</p></li>
<li class="listitem"><p><code class="varname">v.clo &lt;clo_option&gt;...</code> changes one or more
        dynamic command line options. If no clo_option is given, lists the
        dynamically changeable options. See
        <a class="xref" href="manual-core.html#manual-core.dynopts" title="2.7.8. Dynamically Changing Options">Dynamically Change Options</a>.
      </p></li>
<li class="listitem">
<p><code class="varname">v.set {gdb_output | log_output |
    mixed_output}</code> allows redirection of the Valgrind output
    (e.g. the errors detected by the tool).  The default setting is
    <code class="computeroutput">mixed_output</code>.</p>
<p>With <code class="computeroutput">mixed_output</code>, the
    Valgrind output goes to the Valgrind log (typically stderr) while
    the output of the interactive GDB monitor commands (e.g. 
    <code class="computeroutput">v.info last_error</code>)
    is displayed by GDB.</p>
<p>With <code class="computeroutput">gdb_output</code>, both the
    Valgrind output and the interactive GDB monitor commands output are
    displayed by GDB.</p>
<p>With <code class="computeroutput">log_output</code>, both the
    Valgrind output and the interactive GDB monitor commands output go
    to the Valgrind log.</p>
</li>
<li class="listitem"><p><code class="varname">v.wait [ms (default 0)]</code> instructs
    Valgrind gdbserver to sleep "ms" milli-seconds and then
    continue.  When sent from a standalone vgdb, if this is the last
    command, the Valgrind process will continue the execution of the
    guest process. The typical usage of this is to use vgdb to send a
    "no-op" command to a Valgrind gdbserver so as to continue the
    execution of the guest process.
    </p></li>
<li class="listitem"><p><code class="varname">v.kill</code> requests the gdbserver to kill
    the process. This can be used from a standalone vgdb to properly
    kill a Valgrind process which is currently expecting a vgdb
    connection.</p></li>
<li class="listitem"><p><code class="varname">v.set vgdb-error &lt;errornr&gt;</code>
    dynamically changes the value of the 
    <code class="option">--vgdb-error</code> argument. A
    typical usage of this is to start with
    <code class="option">--vgdb-error=0</code> on the
    command line, then set a few breakpoints, set the vgdb-error value
    to a huge value and continue execution.</p></li>
<li class="listitem"><p><code class="varname">xtmemory [&lt;filename&gt; default xtmemory.kcg.%p.%n]</code>
      requests the tool (Memcheck, Massif, Helgrind) to produce an xtree heap memory report. 
      See <a class="xref" href="manual-core.html#manual-core.xtree" title="2.10. Execution Trees">Execution Trees</a> for
      a detailed explanation about execution trees. </p></li>
</ul></div>
<p>The following Valgrind monitor commands are useful for
investigating the behaviour of Valgrind or its gdbserver in case of
problems or bugs.</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><code class="varname">v.do expensive_sanity_check_general</code>
    executes various sanity checks. In particular, the sanity of the
    Valgrind heap is verified. This can be useful if you suspect that
    your program and/or Valgrind has a bug corrupting Valgrind data
    structure.  It can also be used when a Valgrind tool
    reports a client error to the connected GDB, in order to verify
    the sanity of Valgrind before continuing the execution.
    </p></li>
<li class="listitem"><p><code class="varname">v.info gdbserver_status</code> shows the
    gdbserver status. In case of problems (e.g. of communications),
    this shows the values of some relevant Valgrind gdbserver internal
    variables.  Note that the variables related to breakpoints and
    watchpoints (e.g. the number of breakpoint addresses and the number of
    watchpoints) will be zero, as GDB by default removes all
    watchpoints and breakpoints when execution stops, and re-inserts
    them when resuming the execution of the debugged process.  You can
    change this GDB behaviour by using the GDB command
    <code class="computeroutput">set breakpoint always-inserted on</code>.
    </p></li>
<li class="listitem"><p><code class="varname">v.info memory [aspacemgr]</code> shows the statistics of
    Valgrind's internal heap management. If
    option <code class="option">--profile-heap=yes</code> was given, detailed
    statistics will be output. With the optional argument
    <code class="computeroutput">aspacemgr</code>. the segment list maintained
    by valgrind address space manager will be output. Note that
    this list of segments is always output on the Valgrind log.
    </p></li>
<li class="listitem"><p><code class="varname">v.info exectxt</code> shows information about
    the "executable contexts" (i.e. the stack traces) recorded by
    Valgrind.  For some programs, Valgrind can record a very high
    number of such stack traces, causing a high memory usage.  This
    monitor command shows all the recorded stack traces, followed by
    some statistics. This can be used to analyse the reason for having
    a big number of stack traces. Typically, you will use this command
    if <code class="varname">v.info memory</code> has shown significant memory
    usage by the "exectxt" arena.
    </p></li>
<li class="listitem">
<p><code class="varname">v.info scheduler</code> shows various
    information about threads. First, it outputs the host stack trace,
    i.e. the Valgrind code being executed. Then, for each thread, it
    outputs the thread state. For non terminated threads, the state is
    followed by the guest (client) stack trace. Finally, for each
    active thread or for each terminated thread slot not yet re-used,
    it shows the max usage of the valgrind stack.</p>
<p>Showing the client stack traces allows to compare the stack
    traces produced by the Valgrind unwinder with the stack traces
    produced by GDB+Valgrind gdbserver. Pay attention that GDB and
    Valgrind scheduler status have their own thread numbering
    scheme. To make the link between the GDB thread number and the
    corresponding Valgrind scheduler thread number, use the GDB
    command <code class="computeroutput">info threads</code>.  The output
    of this command shows the GDB thread number and the valgrind
    'tid'. The 'tid' is the thread number output
    by <code class="computeroutput">v.info scheduler</code>.  When using
    the callgrind tool, the callgrind monitor command
    <code class="computeroutput">status</code> outputs internal callgrind
    information about the stack/call graph it maintains.
    </p>
</li>
<li class="listitem"><p><code class="varname">v.info stats</code> shows various valgrind core and
    tool statistics. With this, Valgrind and tool statistics can
    be examined while running, even without option <code class="option">--stats=yes</code>.
    </p></li>
<li class="listitem"><p><code class="varname">v.info unwind  &lt;addr&gt; [&lt;len&gt;]</code> shows
    the CFI unwind debug info for the address range [addr, addr+len-1].
    The default value of &lt;len&gt; is 1, giving the unwind information
    for the instruction at &lt;addr&gt;.
    </p></li>
<li class="listitem"><p><code class="varname">v.set debuglog &lt;intvalue&gt;</code> sets the
    Valgrind debug log level to &lt;intvalue&gt;.  This allows to
    dynamically change the log level of Valgrind e.g. when a problem
    is detected.</p></li>
<li class="listitem">
<p><code class="varname">v.set hostvisibility [yes*|no]</code> The value
    "yes" indicates to gdbserver that GDB can look at the Valgrind
    'host' (internal) status/memory. "no" disables this access.
    When hostvisibility is activated, GDB can e.g. look at Valgrind
    global variables. As an example, to examine a Valgrind global
    variable of the memcheck tool on an x86, do the following setup:</p>
<pre class="screen">
(gdb) monitor v.set hostvisibility yes
(gdb) add-symbol-file /path/to/tool/executable/file/memcheck-x86-linux 0x58000000
add symbol table from file "/path/to/tool/executable/file/memcheck-x86-linux" at
	.text_addr = 0x58000000
(y or n) y
Reading symbols from /path/to/tool/executable/file/memcheck-x86-linux...done.
(gdb) 
</pre>
<p>After that, variables defined in memcheck-x86-linux can be accessed, e.g.</p>
<pre class="screen">
(gdb) p /x vgPlain_threads[1].os_state
$3 = {lwpid = 0x4688, threadgroup = 0x4688, parent = 0x0, 
  valgrind_stack_base = 0x62e78000, valgrind_stack_init_SP = 0x62f79fe0, 
  exitcode = 0x0, fatalsig = 0x0}
(gdb) p vex_control
$5 = {iropt_verbosity = 0, iropt_level = 2, 
  iropt_register_updates = VexRegUpdUnwindregsAtMemAccess, 
  iropt_unroll_thresh = 120, guest_max_insns = 60, guest_chase_thresh = 10}
(gdb) 
</pre>
</li>
<li class="listitem">
<p><code class="varname">v.translate &lt;address&gt;
    [&lt;traceflags&gt;]</code> shows the translation of the block
    containing <code class="computeroutput">address</code> with the given
    trace flags. The <code class="computeroutput">traceflags</code> value
    bit patterns have similar meaning to Valgrind's
    <code class="option">--trace-flags</code> option.  It can be given
    in hexadecimal (e.g. 0x20) or decimal (e.g. 32) or in binary 1s
    and 0s bit (e.g. 0b00100000). The default value of the traceflags
    is 0b00100000, corresponding to "show after instrumentation". 
    The output of this command always goes to the Valgrind
    log.</p>
<p>The additional bit flag 0b100000000 (bit 8)
    has no equivalent in the <code class="option">--trace-flags</code> option.
    It enables tracing of the gdbserver specific instrumentation.  Note
    that this bit 8 can only enable the addition of gdbserver
    instrumentation in the trace.  Setting it to 0 will not
    disable the tracing of the gdbserver instrumentation if it is
    active for some other reason, for example because there is a breakpoint at
    this address or because gdbserver is in single stepping
    mode.</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="manual-core-adv.wrapping"></a>3.3. Function wrapping</h2></div></div></div>
<p>
Valgrind allows calls to some specified functions to be intercepted and
rerouted to a different, user-supplied function.  This can do whatever it
likes, typically examining the arguments, calling onwards to the original,
and possibly examining the result.  Any number of functions may be
wrapped.</p>
<p>
Function wrapping is useful for instrumenting an API in some way.  For
example, Helgrind wraps functions in the POSIX pthreads API so it can know
about thread status changes, and the core is able to wrap
functions in the MPI (message-passing) API so it can know
of memory status changes associated with message arrival/departure.
Such information is usually passed to Valgrind by using client
requests in the wrapper functions, although the exact mechanism may vary.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.example"></a>3.3.1. A Simple Example</h3></div></div></div>
<p>Supposing we want to wrap some function</p>
<pre class="programlisting">
int foo ( int x, int y ) { return x + y; }</pre>
<p>A wrapper is a function of identical type, but with a special name
which identifies it as the wrapper for <code class="computeroutput">foo</code>.
Wrappers need to include
supporting macros from <code class="filename">valgrind.h</code>.
Here is a simple wrapper which prints the arguments and return value:</p>
<pre class="programlisting">
#include &lt;stdio.h&gt;
#include "valgrind.h"
int I_WRAP_SONAME_FNNAME_ZU(NONE,foo)( int x, int y )
{
   int    result;
   OrigFn fn;
   VALGRIND_GET_ORIG_FN(fn);
   printf("foo's wrapper: args %d %d\n", x, y);
   CALL_FN_W_WW(result, fn, x,y);
   printf("foo's wrapper: result %d\n", result);
   return result;
}
</pre>
<p>To become active, the wrapper merely needs to be present in a text
section somewhere in the same process' address space as the function
it wraps, and for its ELF symbol name to be visible to Valgrind.  In
practice, this means either compiling to a 
<code class="computeroutput">.o</code> and linking it in, or
compiling to a <code class="computeroutput">.so</code> and 
<code class="computeroutput">LD_PRELOAD</code>ing it in.  The latter is more
convenient in that it doesn't require relinking.</p>
<p>All wrappers have approximately the above form.  There are three
crucial macros:</p>
<p><code class="computeroutput">I_WRAP_SONAME_FNNAME_ZU</code>: 
this generates the real name of the wrapper.
This is an encoded name which Valgrind notices when reading symbol
table information.  What it says is: I am the wrapper for any function
named <code class="computeroutput">foo</code> which is found in 
an ELF shared object with an empty
("<code class="computeroutput">NONE</code>") soname field.  The specification 
mechanism is powerful in
that wildcards are allowed for both sonames and function names.  
The details are discussed below.</p>
<p><code class="computeroutput">VALGRIND_GET_ORIG_FN</code>: 
once in the wrapper, the first priority is
to get hold of the address of the original (and any other supporting
information needed).  This is stored in a value of opaque 
type <code class="computeroutput">OrigFn</code>.
The information is acquired using 
<code class="computeroutput">VALGRIND_GET_ORIG_FN</code>.  It is crucial
to make this macro call before calling any other wrapped function
in the same thread.</p>
<p><code class="computeroutput">CALL_FN_W_WW</code>: eventually we will
want to call the function being
wrapped.  Calling it directly does not work, since that just gets us
back to the wrapper and leads to an infinite loop.  Instead, the result
lvalue, 
<code class="computeroutput">OrigFn</code> and arguments are
handed to one of a family of macros of the form 
<code class="computeroutput">CALL_FN_*</code>.  These
cause Valgrind to call the original and avoid recursion back to the
wrapper.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.specs"></a>3.3.2. Wrapping Specifications</h3></div></div></div>
<p>This scheme has the advantage of being self-contained.  A library of
wrappers can be compiled to object code in the normal way, and does
not rely on an external script telling Valgrind which wrappers pertain
to which originals.</p>
<p>Each wrapper has a name which, in the most general case says: I am the
wrapper for any function whose name matches FNPATT and whose ELF
"soname" matches SOPATT.  Both FNPATT and SOPATT may contain wildcards
(asterisks) and other characters (spaces, dots, @, etc) which are not 
generally regarded as valid C identifier names.</p>
<p>This flexibility is needed to write robust wrappers for POSIX pthread
functions, where typically we are not completely sure of either the
function name or the soname, or alternatively we want to wrap a whole
set of functions at once.</p>
<p>For example, <code class="computeroutput">pthread_create</code> 
in GNU libpthread is usually a
versioned symbol - one whose name ends in, eg, 
<code class="computeroutput">@GLIBC_2.3</code>.  Hence we
are not sure what its real name is.  We also want to cover any soname
of the form <code class="computeroutput">libpthread.so*</code>.
So the header of the wrapper will be</p>
<pre class="programlisting">
int I_WRAP_SONAME_FNNAME_ZZ(libpthreadZdsoZd0,pthreadZucreateZAZa)
  ( ... formals ... )
  { ... body ... }
</pre>
<p>In order to write unusual characters as valid C function names, a
Z-encoding scheme is used.  Names are written literally, except that
a capital Z acts as an escape character, with the following encoding:</p>
<pre class="programlisting">
     Za   encodes    *
     Zp              +
     Zc              :
     Zd              .
     Zu              _
     Zh              -
     Zs              (space)
     ZA              @
     ZZ              Z
     ZL              (       # only in valgrind 3.3.0 and later
     ZR              )       # only in valgrind 3.3.0 and later
</pre>
<p>Hence <code class="computeroutput">libpthreadZdsoZd0</code> is an 
encoding of the soname <code class="computeroutput">libpthread.so.0</code>
and <code class="computeroutput">pthreadZucreateZAZa</code> is an encoding 
of the function name <code class="computeroutput">pthread_create@*</code>.
</p>
<p>The macro <code class="computeroutput">I_WRAP_SONAME_FNNAME_ZZ</code> 
constructs a wrapper name in which
both the soname (first component) and function name (second component)
are Z-encoded.  Encoding the function name can be tiresome and is
often unnecessary, so a second macro,
<code class="computeroutput">I_WRAP_SONAME_FNNAME_ZU</code>, can be
used instead.  The <code class="computeroutput">_ZU</code> variant is 
also useful for writing wrappers for
C++ functions, in which the function name is usually already mangled
using some other convention in which Z plays an important role.  Having
to encode a second time quickly becomes confusing.</p>
<p>Since the function name field may contain wildcards, it can be
anything, including just <code class="computeroutput">*</code>.
The same is true for the soname.
However, some ELF objects - specifically, main executables - do not
have sonames.  Any object lacking a soname is treated as if its soname
was <code class="computeroutput">NONE</code>, which is why the original 
example above had a name
<code class="computeroutput">I_WRAP_SONAME_FNNAME_ZU(NONE,foo)</code>.</p>
<p>Note that the soname of an ELF object is not the same as its
file name, although it is often similar.  You can find the soname of
an object <code class="computeroutput">libfoo.so</code> using the command
<code class="computeroutput">readelf -a libfoo.so | grep soname</code>.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.semantics"></a>3.3.3. Wrapping Semantics</h3></div></div></div>
<p>The ability for a wrapper to replace an infinite family of functions
is powerful but brings complications in situations where ELF objects
appear and disappear (are dlopen'd and dlclose'd) on the fly.
Valgrind tries to maintain sensible behaviour in such situations.</p>
<p>For example, suppose a process has dlopened (an ELF object with
soname) <code class="filename">object1.so</code>, which contains 
<code class="computeroutput">function1</code>.  It starts to use
<code class="computeroutput">function1</code> immediately.</p>
<p>After a while it dlopens <code class="filename">wrappers.so</code>,
which contains a wrapper
for <code class="computeroutput">function1</code> in (soname) 
<code class="filename">object1.so</code>.  All subsequent calls to 
<code class="computeroutput">function1</code> are rerouted to the wrapper.</p>
<p>If <code class="filename">wrappers.so</code> is 
later dlclose'd, calls to <code class="computeroutput">function1</code> are 
naturally routed back to the original.</p>
<p>Alternatively, if <code class="filename">object1.so</code>
is dlclose'd but <code class="filename">wrappers.so</code> remains,
then the wrapper exported by <code class="filename">wrappers.so</code>
becomes inactive, since there
is no way to get to it - there is no original to call any more.  However,
Valgrind remembers that the wrapper is still present.  If 
<code class="filename">object1.so</code> is
eventually dlopen'd again, the wrapper will become active again.</p>
<p>In short, valgrind inspects all code loading/unloading events to
ensure that the set of currently active wrappers remains consistent.</p>
<p>A second possible problem is that of conflicting wrappers.  It is 
easily possible to load two or more wrappers, both of which claim
to be wrappers for some third function.  In such cases Valgrind will
complain about conflicting wrappers when the second one appears, and
will honour only the first one.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.debugging"></a>3.3.4. Debugging</h3></div></div></div>
<p>Figuring out what's going on given the dynamic nature of wrapping
can be difficult.  The 
<code class="option">--trace-redir=yes</code> option makes 
this possible
by showing the complete state of the redirection subsystem after
every
<code class="function">mmap</code>/<code class="function">munmap</code>
event affecting code (text).</p>
<p>There are two central concepts:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>A "redirection specification" is a binding of 
  a (soname pattern, fnname pattern) pair to a code address.
  These bindings are created by writing functions with names
  made with the 
  <code class="computeroutput">I_WRAP_SONAME_FNNAME_{ZZ,_ZU}</code>
  macros.</p></li>
<li class="listitem"><p>An "active redirection" is a code-address to 
  code-address binding currently in effect.</p></li>
</ul></div>
<p>The state of the wrapping-and-redirection subsystem comprises a set of
specifications and a set of active bindings.  The specifications are
acquired/discarded by watching all 
<code class="function">mmap</code>/<code class="function">munmap</code>
events on code (text)
sections.  The active binding set is (conceptually) recomputed from
the specifications, and all known symbol names, following any change
to the specification set.</p>
<p><code class="option">--trace-redir=yes</code> shows the contents 
of both sets following any such event.</p>
<p><code class="option">-v</code> prints a line of text each 
time an active specification is used for the first time.</p>
<p>Hence for maximum debugging effectiveness you will need to use both
options.</p>
<p>One final comment.  The function-wrapping facility is closely
tied to Valgrind's ability to replace (redirect) specified
functions, for example to redirect calls to 
<code class="function">malloc</code> to its
own implementation.  Indeed, a replacement function can be
regarded as a wrapper function which does not call the original.
However, to make the implementation more robust, the two kinds
of interception (wrapping vs replacement) are treated differently.
</p>
<p><code class="option">--trace-redir=yes</code> shows 
specifications and bindings for both
replacement and wrapper functions.  To differentiate the 
two, replacement bindings are printed using 
<code class="computeroutput">R-&gt;</code> whereas 
wraps are printed using <code class="computeroutput">W-&gt;</code>.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.limitations-cf"></a>3.3.5. Limitations - control flow</h3></div></div></div>
<p>For the most part, the function wrapping implementation is robust.
The only important caveat is: in a wrapper, get hold of
the <code class="computeroutput">OrigFn</code> information using 
<code class="computeroutput">VALGRIND_GET_ORIG_FN</code> before calling any
other wrapped function.  Once you have the 
<code class="computeroutput">OrigFn</code>, arbitrary
calls between, recursion between, and longjumps out of wrappers
should work correctly.  There is never any interaction between wrapped
functions and merely replaced functions 
(eg <code class="function">malloc</code>), so you can call
<code class="function">malloc</code> etc safely from within wrappers.
</p>
<p>The above comments are true for {x86,amd64,ppc32,arm,mips32,s390}-linux.
On
ppc64-linux function wrapping is more fragile due to the (arguably
poorly designed) ppc64-linux ABI.  This mandates the use of a shadow
stack which tracks entries/exits of both wrapper and replacement
functions.  This gives two limitations: firstly, longjumping out of
wrappers will rapidly lead to disaster, since the shadow stack will
not get correctly cleared.  Secondly, since the shadow stack has
finite size, recursion between wrapper/replacement functions is only
possible to a limited depth, beyond which Valgrind has to abort the
run.  This depth is currently 16 calls.</p>
<p>For all platforms ({x86,amd64,ppc32,ppc64,arm,mips32,s390}-linux)
all the above
comments apply on a per-thread basis.  In other words, wrapping is
thread-safe: each thread must individually observe the above
restrictions, but there is no need for any kind of inter-thread
cooperation.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.limitations-sigs"></a>3.3.6. Limitations - original function signatures</h3></div></div></div>
<p>As shown in the above example, to call the original you must use a
macro of the form <code class="computeroutput">CALL_FN_*</code>.  
For technical reasons it is impossible
to create a single macro to deal with all argument types and numbers,
so a family of macros covering the most common cases is supplied.  In
what follows, 'W' denotes a machine-word-typed value (a pointer or a
C <code class="computeroutput">long</code>), 
and 'v' denotes C's <code class="computeroutput">void</code> type.
The currently available macros are:</p>
<pre class="programlisting">
CALL_FN_v_v    -- call an original of type  void fn ( void )
CALL_FN_W_v    -- call an original of type  long fn ( void )

CALL_FN_v_W    -- call an original of type  void fn ( long )
CALL_FN_W_W    -- call an original of type  long fn ( long )

CALL_FN_v_WW   -- call an original of type  void fn ( long, long )
CALL_FN_W_WW   -- call an original of type  long fn ( long, long )

CALL_FN_v_WWW  -- call an original of type  void fn ( long, long, long )
CALL_FN_W_WWW  -- call an original of type  long fn ( long, long, long )

CALL_FN_W_WWWW -- call an original of type  long fn ( long, long, long, long )
CALL_FN_W_5W   -- call an original of type  long fn ( long, long, long, long, long )
CALL_FN_W_6W   -- call an original of type  long fn ( long, long, long, long, long, long )
and so on, up to 
CALL_FN_W_12W
</pre>
<p>The set of supported types can be expanded as needed.  It is
regrettable that this limitation exists.  Function wrapping has proven
difficult to implement, with a certain apparently unavoidable level of
ickiness.  After several implementation attempts, the present
arrangement appears to be the least-worst tradeoff.  At least it works
reliably in the presence of dynamic linking and dynamic code
loading/unloading.</p>
<p>You should not attempt to wrap a function of one type signature with a
wrapper of a different type signature.  Such trickery will surely lead
to crashes or strange behaviour.  This is not a limitation
of the function wrapping implementation, merely a reflection of the
fact that it gives you sweeping powers to shoot yourself in the foot
if you are not careful.  Imagine the instant havoc you could wreak by
writing a wrapper which matched any function name in any soname - in
effect, one which claimed to be a wrapper for all functions in the
process.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="manual-core-adv.wrapping.examples"></a>3.3.7. Examples</h3></div></div></div>
<p>In the source tree, 
<code class="filename">memcheck/tests/wrap[1-8].c</code> provide a series of
examples, ranging from very simple to quite advanced.</p>
<p><code class="filename">mpi/libmpiwrap.c</code> is an example 
of wrapping a big, complex API (the MPI-2 interface).  This file defines 
almost 300 different wrappers.</p>
</div>
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